Due to the asynchronous nature of e-mail systems, it is possible for an e-mail message to not be successfully delivered to the intended recipient. The unsuccessful delivery of an e-mail message can occur for many reasons. For example, the e-mail message may be too large for the recipient's mailbox, or the recipient's mailbox may be full. In addition, embarrassing and costly mistakes can be made when composing e-mail messages. For example, a private e-mail may be inadvertently sent to a large audience, or confidential material may be inadvertently sent to external recipients.
These problems occur because information about the e-mail system and about the recipients of an e-mail message are not available to the sender while the message is being composed. It is only after sending the message that the problem is discovered.